Solder joints are a critical element of surface mount attachment of electronic components. Previously, continuous process quality of SMT circuit board assembly was monitored using visual inspection, pull tests, and periodic Accelerated Thermal Cycling (ATC). Visual inspection is not sufficient because many defects are not visually observable. For example, joints of grid array surface mount components can not be readily observed. In pull testing, assembled components are pulled off the circuit board and the appearance of the fractured joints indicate the quality of the assembly process. Again, this method does not disclose all the defects because it does not replicate field failure mechanisms. Both visual inspection and pull testing require the services of a skilled operator for extensive periods of time for each board assembly tested. ATC is a good method of finding defects which could cause field failures Also, ATC testing can be automated so a skilled operator is only required to electrically test the board after cycling. However, ATC is not a practical testing method for monitoring continuous process quality because the total test time is too long. Typically, ATC data is not available until weeks after production.
Cyclic bending has been used to test the quality of soldered joints in circuit board assemblies. Japanese Patent JP 03-245600 discloses "a test device to check a printed board in soldering quality." The test uses "a probe after the printed circuit board is subjected to a prescribed frequency of bending tests." In Soviet Union patent SU 1723679-A1 constant bending upward and downward is applied to a PCB for "non-destructive quality control testing of metalizations and contact junctions."
Cyclic bending fatigue testing has also been used in other fields for determining bulk fatigue properties of many materials. For example U.S. Pat. No. 3,381,526 to Rastogi et al discloses a method and machine to perform such fatigue tests on cantilever beams with a waist section. Manahan et al in U.S. Pat. No. 4,567,774 discloses "a miniaturized bend test of specimens" and the mechanical behavior of the material is determined from the measurement taken during the "bending of the specimen and is processed according to the principles of linear or non-linear material mechanics;" and in U.S. Pat. No. 4,895,027 discloses a method for determining "plane strain fracture toughness, dynamic plane strain crack initiation and arrest fracture toughness" and other fracture characteristics. U.S. Pat. No. 5,184,517 to Kelzer discloses a "test fixture and test method is designed to establish statistical information concerning the breaking force for deflection of a printed circuit board when the print circuit board is broken along pre-existing score lines." U.S. Pat. No. 5,079,955 to Eberhardt discloses "A fatigue testing system subjects a test specimen to a rotary stress applied about an axis at one or both ends which is perpendicular to the axis of the test selected specimen" and is used to detect "stress failure" for tested materials.
Torsional testing has also been used to evaluate mechanical properties of materials and structures. In U.S. Pat. No. 4,958,522 McKinlay discloses a sample "is held between two axially aligned jaws. . . The sample is subjected to a twisting force by rotation of one of the jaws and the force and angle of deflection are measured. This gives a relative determination of the board's structural property and is used to assess damage to the corrugated medium during corrugation and subsequent processing steps such as printing."
All the above citations are hereby incorporated by reference to provide an enabling disclosure and to support the claims to which applicant is entitled by law.